Communication devices such as wireless device are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been developed in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
For mobility reasons where a UE in a LTE network performs e.g. a handover from a cell served by an eNB to another cell served by another eNB, the UE typically performs measurements on signals or channels. There are defined different types of parameters that a UE may measure. For example, in the LTE network, a UE may measure on reference signal: RSRP (Reference Signal Received Power) and RSRQ (Reference Signal Received Quality).
RSRP (Reference Signal Receive Power) is the average power of Resource Elements (RE) that carry cell specific Reference Signals (CRS) over the entire system bandwidth. RSRP is measured in the symbols carrying the reference signal. Its typical range is around −44 to −130 dBm. This measurement is used in Radio Resource Control (RRC) Idle/Connected, cell se-selection/selection, handover scenarios etc. Since a UE for RSRP measures only the reference power, this measure may be viewed as the strength of the wanted/desired signal. However, the RSRP does not give information about the signal/channel quality. In other words, RSRP provides the signal strength of the desired signal, not the quality of the signal. For quality of the signal information another parameter called ‘RSRQ’ is used.
RSRQ (Reference Signal Received Quality) is defined as (N×RSRP)/RSSI, where N is the number of Resource Blocks (RBs) over the measurement bandwidth. This is not the direct measurement. It is a kind of derived value from RSRP and RSSI (Received Signal Strength Indicator). By dividing RSRP by RSSI, it could give some information about interference as well in addition to the strength of the wanted/desired signal. The RSSI parameter represents the entire received power including the wanted power from the serving cell or serving eNB as well as all co-channel power and other sources of noise. Measuring RSRQ becomes important near the cell edge when decisions need to be made, regardless of absolute RSRP, to perform a handover to the next cell. It should be mentioned that RSRQ is used only during connected states. Intra- and inter-frequency absolute RSRQ accuracy varies from ±2.5 to ±4 dB, which is similar to the inter frequency relative RSRQ accuracy of ±3 to ±4 dB.
The RSSI is a parameter which provides information about total received wide-band power (measure in all symbols) including all interference and thermal noise. In other words, the RSSI is the total power a UE observes across the whole band. This includes the main signal and co-channel non-serving cell signal, adjacent channel interference and even the thermal noise within the specified band. This is the power of non-demodulated signal, so a UE may measure this power without any synchronization and demodulation. So in LTE RSRP provides information about signal strength and RSSI helps in determining interference and noise information. This is the reason RSRQ measurement and calculation is based on both RSRP and RSSI.
As mentioned above, the link quality (RSRQ) measurement aims at providing an indirect indication of the expected Signal to Interference Noise Ratio (SINR) for a given LTE cell. The RSRQ is a function of RSRP and RSSI as previously described. Both terms are determined/calculated based on the DownLink (DL) data channel Physical Downlink Shared Channel (PDSCH), since it is interesting for the measuring UE to obtain information about DL link quality. Since the position of CRS used for RSRP calculation and the LTE frame structure are known to the UE receiver either by specification or via broadcast system information, the UE receiver has no ambiguity in selecting the correct Resource Elements (REs) to be used for RSRP and RSSI measurements.
However, differently from the cellular case (LTE case above), a Device-to-Device (D2D) capable UE receiver is in general not aware of the position of reference signals and D2D data transmissions for RSRP and RSSI estimation. This is because, differently from DL from a eNB, D2D UE devices do not in general transmit regular signals such as CRS in D2D communications. Therefore, if the legacy procedures (i.e. from cellular network procedures) are applied directly to D2D, there is a risk to either calculate RSRP erroneously (due to lack of data transmission) or to include the incorrect interference contribution in the RSSI calculation.
A short technical description of D2D is presented below.
D2D communications (also defined as Proximity Service (ProSe) direct communication or sidelink communication or peer to peer communication, etc) as as an underlay to cellular networks have been proposed as a means to take advantage of the proximity of communicating devices (UEs) and at the same time to allow devices to operate in a controlled interference environment. It is suggested that such D2D communication shares the same spectrum as the cellular system, for example by reserving some of the cellular uplink resources for D2D purposes. Allocating dedicated spectrum for D2D purposes is a less likely alternative as spectrum is a scarce resource and (dynamic) sharing between the D2D services and cellular services is more flexible and provides higher spectrum efficiency.
The transmission mode when sending data during D2D communication may be either:                Unicast—a specific UE is the receiver        Multicast (may also be denoted group-cast)—a group of UEs are receivers        Broadcast—all UEs are receivers        
With connectionless D2D communication, data can be sent from one device or UE to another device or UE without prior arrangement, thereby reducing the overhead and increasing the communication capacity which is crucial in emergency situations. The source device transmits data to one or more devices, without first ensuring that the recipients are available and ready to receive the data. Connectionless communication may be used for one-to-one or one-to-many communication, but it is particularly effective for multicast and broadcast transmissions and thus well-suited for broadcast and group communication.
When a D2D UE is in network coverage of a eNB, any D2D communication is controlled by the network node (such as the eNB). Since the radio resources in a cell (especially for the uplink resources) are shared between traditional cellular communication and D2D communication, the eNB should divide and assign the radio resources also in case of D2D communication, in case the UEs are in coverage. In 3GPP Release 12, the ProSe or D2D UE Information message has been introduced as part of the RRC protocol. This information message is used whenever the UE needs to inform the eNB about a need to ProSe communication or ProSe Discovery. For communication, the information message contains a list of ProSe destinations, and an index associated to each of these. In case of multicast communication a ProSe destination is a ProSe Layer 2 Group identity and for unicast communication it is a ProSe UE Identity. The index may be used as a 4 bit short reference to a given group or unicast destination, e.g. as used in the MAC Buffer Status Report when transmitting data to the destination.
Moreover, a given unicast traffic session between two UEs may use either a direct communication path or the infrastructure communication path. When using the direct communication path, the data is transmitted directly between the UEs, using D2D communication. On the other hand, when using the infrastructure communication path, the data is instead transmitted via the network nodes. The latter case is only available when both UEs are in coverage by the network node.
A service continuity switch is the procedure to move a user traffic session from the direct communication D2D path to the infrastructure communication path, or vice versa. With “infrastructure communication path” we mean that the packets use the non-D2D, legacy, physical (uplink and downlink) channels and also that the packets are transmitted over a bearer, which is a tunnel between the UE and the Packet Data Network GateWay (PDN GW) node.
Service continuity between infrastructure and ProSe Direct Communication paths maybe divided into two different scenarios:
Scenario 1 (“One UE”) is shown in FIG. 1 wherein a user or UE traffic session is maintained even when a UE goes between in coverage and out of coverage. In this scenario, the mobility is limited to one UE (UE1), and the other UE (UE2) acts as the relay between the remote UE and NW (eNB). The switch and eNB cell boarder is also shown.
“Scenario 2 (“Two UEs”) is shown in FIG. 2 where two switches are shown: A switch into ProSe D2D communication path between two in-coverage UEs when they come within proximity of each other. In this scenario, the mobility of both UEs (UE1 and UE2) would be considered. Two eNBs, eNB1 and eNB2 are shown where here, UE1 is considered served by eNB1 and UE2 served by eNB2. Hence a switch is performed from the infrastructure mode to the direct D2D mode i.e. between the UEs.
Radio-proximity related measurements between D2D devices may be exploited in the above scenarios for determining the switching of the path e.g. from the infrastructure mode to a direct D2D mode. However, as explained before, differently from the cellular case (LTE), a D2D or ProSe capable UE receiver is in general not aware of the position of reference signals and D2D data transmissions for RSRP and RSSI estimation. This is because, differently from DL from a eNB, D2D UE devices do not in general transmit regular signals such as CRS in D2D communications. Therefore, if the legacy measurement procedures (i.e. from cellular network procedures) are applied directly to D2D, there is a risk to either determine or calculate RSRP erroneously (due to lack of data transmission) or to include the incorrect interference contribution in the RSSI calculation or determination. This would affect the performance of D2D communications in the different scenarios described before.